diff --git a/asfsmd/_utils.py b/asfsmd/_utils.py
new file mode 100644
index 0000000..aada29f
--- /dev/null
+++ b/asfsmd/_utils.py
@@ -0,0 +1,14 @@
+"""Utility functions for asfsmd."""
+
+from typing import Any, Iterable, List
+
+
+def unique(data: Iterable[Any]) -> List[Any]:
+    """Return a list of unique items preserving the input ordering."""
+    unique_items = []
+    unique_items_set = set()
+    for item in data:
+        if item not in unique_items_set:
+            unique_items.append(item)
+            unique_items_set.add(item)
+    return unique_items
diff --git a/tests/test__utils.py b/tests/test__utils.py
new file mode 100644
index 0000000..f8c118a
--- /dev/null
+++ b/tests/test__utils.py
@@ -0,0 +1,23 @@
+"""Unit tests for the `asfsmd._utils` module."""
+
+import itertools
+from asfsmd._utils import unique
+
+import pytest
+
+
+@pytest.mark.parametrize(
+    ["in_", "out"],
+    [
+        pytest.param(["a", "b", "c"], ["a", "b", "c"], id="unique-list"),
+        pytest.param(["a", "b", "c", "b"], ["a", "b", "c"], id="list"),
+        pytest.param((1, 2, 2, 3, 1, 2), [1, 2, 3], id="tuple"),
+        pytest.param(range(3), [0, 1, 2], id="generator"),
+        pytest.param(
+            itertools.chain(range(3, 0, -1), range(3)),
+            [3, 2, 1, 0],
+            id="reversed-generator"),
+    ],
+)
+def test_unique(in_, out):
+    assert unique(in_) == out